Introduction to Quantum Computing


Why quantum: How big is a bit?


In this lesson, you will predict the size of a single piece of information storage.


1)      What is a bit?

Have a look at the Wikipedia page:

A bit is a binary digit, and can be in only one of two states at a time: the two states are most commonly represented as either a 0 or 1. It stores the smallest possible piece of information in a computer.

The international symbol for ‘power’ includes the number ‘1’ (one) and ‘0’ (zero) within it.


2)      How do you build a bit?

Well, you could construct a computer in Minecraft. See YouTube videos: [16 bit computer by Sw1Ftx16]

In the real world, here are some ways in which manufacturers build storage elements for computers:

This one shows how to use an existing electronic chip (integrated circuit) -

The integrated circuits that make computer storage chips are known as RAM (random access memory). Here is a factory tour:

To understand how a RAM chip works, please see:

You might see that bits are usually stored in groups of 8. A group of 8 bits is usually called a byte.


3)      Webquest to predict bit sizes in the future

Bits come in different sizes. Let’s look at how big a bit can be, by looking at computer storage devise in the past and future. Look at these websites to complete the table.

HINT: to work out the size of one bit, find the volume of each storage device, and divide by the number of bits it can store.


Commercially available storage device

Length (mm)

Width (mm)


Volume (mm3)

Storage capacity (bits) (multiply by 8 if stated as bytes or characters)

Volume of each bit (mm3)


Selectron Tube








Mercury delay line





1600 x 8 = ??



Magnetic Core








Hard Disk





5,000,000 x 8



Dynamic RAM solid state integrated circuit Intel 1103








Compact-Disc CD-ROM








Secure Digital flash memory card






64,000,000 x 8



micro-SD card







???? (you predict)

single-atom memory






1 atom = 9.97x10-21

If you put this into a spreadsheet, remember that big and small numbers are expressed in ‘exponential notation’. So, 5x103 (5,000) is written as 5E3 in Excel. Format cells as ‘scientific’ to see this notation. You may find it easier to see the graph if you take the logarithm of the bit-volumes. In that case, the log of the volume of one atom would be -20, so can you predict in what year bit sizes might get there?


4)      Conclusion and check quiz

When you have plotted the size of a bit against the invention year, you should be able to predict when a bit will be as small as an atom (9.97x10-21 mm3).

Others have done similar predictions. You can find many of them on the internet, perhaps at Singularity.

When a ‘bit’ gets as small as a single atom, we are talking about quantum computing.


Figure 1: Single Atom in Ion Trap



Find out if your prediction conforms with Moore’s Law.

Figure 2: A closer look.

When illuminated by a laser of the right blue-violet colour, the atom absorbs and re-emits light particles sufficiently quickly for an ordinary camera to capture it in a long exposure photograph.

The first ever photograph of light as both a particle and wave

Figure 3: The first ever photograph of light as both a particle and wave

 There is a quiz on the contents of this lesson. Click the button below to take the quiz.

Check quiz: How big is a bit?







Figure 1: Single Atom in Ion Trap (David Nadlinger/University of Oxford/EPSRC)

Figure 2: A closer look. (David Nadlinger/University of Oxford/EPSRC)

Figure 3: The first ever photograph of light as both a particle and wave.